With an ever increasing number of applications for electronics, surface mount technology has been used more frequently to construct electronic circuits. In surface mount technology, electronic components typically take the form of chip components having terminals for attachment by soldering to electronic boards. The terminals of the chip components require good solderability while also having heat resistance to withstand high temperatures during soldering and/or operation of the components.
Surface mount capacitors are generally protected from destructive effects of moisture and oxygen found in ambient environment by employing a seal formed by adhesion of a molding resin to a portion of a lead frame for the capacitor. As examples of conventional surface mount capacitors, FIG. 1A shows a cross-sectional view of a first known surface mountable solid electrolytic capacitor, shown generally at 20, and FIG. 1B shows a cross-sectional view of a second known surface mountable solid electrolytic capacitor 30. The known capacitors 20, 30 are formed by molding an interior capacitor element, shown generally at 22, together with lead frames 24 that eventually serve as terminals, with an encapsulating resin 26. In accordance with the first known capacitor 20, the capacitor element is connected to the lead frame 24 by welding an anode stack, shown generally at 21, to an anode terminal 23 and adhering a cathode-coated anode coupon, shown generally at 25, to a cathode terminal 27 using silver loaded adhesive 29, for example. The welding is accomplished using resistance or laser welding. In accordance with the second known capacitor 30, one portion of the lead frame 24 is connected to a capacitor anode riser wire 32, and another portion of the lead frame 24 is connected to a capacitor cathode-coated anode body 34. The capacitor element is coated with a conductive silver paint 36 prior to attaching the lead frame 24 to the cathode-coated anode 34. Unfortunately, oxygen and moisture from the ambient environment may access the interior capacitor element 22 underneath the encapsulating resin 26 to degrade performance of conventional capacitors 20, 30. The oxygen and moisture access commonly occurs following surface mounting of the chip capacitor because of gaps created between the encapsulating resin and the lead frame from heat applied during surface mounting. Alternatively, such gaps may be created during operation of the conventional capacitors 20, 30.
U.S. Pat. No. 6,229,687 (the '687 patent) proposes a method of preventing environment moisture and oxygen from accessing the capacitor element underneath the encapsulated resin. The '687 patent describes a solid electrolytic capacitor having an element that is prepared by forming an oxide layer, a conductive high polymer layer, and a conductor layer in succession on a valve metal. Lead frames of a selected metal are electrically connected to the valve metal and conductor layer, and a protective resin encapsulates the capacitor element and part of the lead frames. The surface of the lead frames contacting the protective resin has a roughened copper layer, and the protective resin has a thermal expansion coefficient that is close to the thermal expansion coefficient of the metal of the lead frames. The copper layer requires additional process steps and associated costs for forming the copper layer and sand blasting the layer for correct roughness.
A need exists for a method of preventing environment moisture and oxygen from accessing interior elements of surface mount components having resin encapsulation. More particularly, a need exists for a method of protecting resin-encapsulated capacitors from moisture and oxygen degradation of capacitor elements underneath the resin encapsulation that is relatively simple and inexpensive.